Hi Pete,
The specific JavaScript needed to achieve what you’re after with this theme is advanced and goes beyond the type of support that this forum is intended for. I can give you some pointers for getting started if you’d like to experiment with solutions, though.
As a starting point, you can open the /js/navigation.js file in your parent theme to see how Cerauno handles dropdown menus in your navigation on mobile by default.
You can then compare this to the code for other themes that do have clickable dropdown menus on mobile. Twenty Fifteen and Twenty Sixteen are two examples of such themes and, as they’re default themes, these will be preinstalled in your setup of WordPress. You’re free to browse through the code for both of those themes.
In terms of using JavaScript with your child theme, you can find guidance here:
https://wordpress.stackexchange.com/questions/26822/how-to-override-javascript-files-in-child-theme
If you’re not that comfortable with JavaScript, you could consider switching to a theme that has menus that behave in the way you like by default and using that as your parent theme instead.
Let me know how you decide to go ahead.